A base station and a mobile station perform a random access procedure to set up an initial call in a circuit-based mobile communication system including WCDMA (wideband code division multiple access).
The random access procedure starts when a mobile station that attempts to access a base station transmits a signal sequence having a specific pattern. The base station manages the signal sequence information having a specific pattern (hereinafter called a “preamble”) based on contention so that any mobile station can use it. Preambles are designed to be detected easily without pre-information. For this purpose, indexes may be used to identify preamble patterns. Information on these preambles is pre-fixed by a system and then used, or is obtained from system information broadcasted from a base station.
In order to perform the random access procedure, the mobile station selects a preamble pattern (or an index) from among information on preambles, generates a preamble based on the selected preamble pattern, and transmits the same to the base station based on contention. The preamble is transmitted during an access slot interval having a predetermined length, and the mobile station transmits the selected preamble pattern during an initial interval of the access slot interval.
The base station transmits response information using a pattern or an index of a corresponding preamble to the mobile station when the preamble is detected within a specific interval, and then a call set-up procedure is initiated. The random access procedure is performed by a mobile station that is in a state (e.g., idle state) of having no connection with the base station.
The random access procedure is performed as described above in a packet switch-based mobile communication system.
However, in the packet switch-based mobile communication system, even if a mobile station is in an active state in which packet service is provided between the mobile station and the base station, a radio resource for packet information communication is not dedicatedly allocated to the mobile station while the packet service is provided. That is, logical channels continue to exist so as to transmit packet data, while physical channels, that is, radio resources, are shared by a plurality of mobile stations that are located within service coverage of the base station.
Accordingly, the mobile station should perform the random access procedure to obtain the uplink radio resource when uplink synchronization is not maintained even though the mobile station is in an RRC (radio resource control)-connected state with the base station.
However, the random access procedure is contention-based, and thereby collision occurs. As a result, it causes delay in performing the random access procedure. Further, it causes delay in allocating radio resources.